Purple: Inactivity has nothing to do with payment. If they don't want to play a certain game type, then that surelly means they probably don't bother their rating in that game either.
Take a look at Fencer's profile. It's been ages since his last Horde Chess game so, naturally, he'd fall off the ratings. He probably doesn't bother that much since he's only playeda few games anyway.
Besides, you can allways come back to the ratings as soon as you finish another game of that type, nomatter how much time there is left in your membership.

(Sorry to use your profile Fencer, but Purple had no suitable example in his )